Conventional image pickup devices use a stepping motor, a voice coil motor and the like for driving a focus lens, and include a control device for controlling them. Upon driving the focus lens by the stepping motor and the like, the control device exhibits hysteresis characteristics in that the position of the focus lens varies with drive directions. In a patent document 1, therefore, focus suffering from hysteresis characteristics is corrected by driving a focus lens in a U-turn such that a drive direction in which a maximum focus evaluation value has been detected coincides with a drive direction for moving the focus lens to a focus position, and detecting a maxim focus evaluation value again. This prevents a control device from being affected by hysteresis characteristics in the patent document 1.
Also, a control device for an image pickup device disclosed in a patent document 2 includes a faint vibration waveform generator for generating a signal that faintly vibrates a movable part of a voice coil motor steadily, and an amplitude detector for detecting the amplitude of the movable part. This control device adjusts the amplitude of the signal output from the faint vibration waveform generator based on the output from the amplitude detector, and keeps the amplitude of the movable part constant. This improves the accuracy of focus detection even with load variation.
Patent document 1: Japanese Patent Application Laid-Open No. 8-186752 (p. 11, FIG. 12)
Patent document 2: Japanese Patent Application Laid-Open No. 8-98575 (p. 6, FIG. 1)
However, it takes time to attain focus with the control device for a voice coil motor and the like disclosed in the patent document 1 because the focus lens always has to coincide with the drive direction in which a maximum focus evaluation value has been detected, and the lens is stopped upon confirming a maximum focus evaluation value again, in order to avoid the effects of hysteresis characteristics.
In addition, the control device that faintly vibrates the movable part of the voice coil motor disclosed in the patent document 2 requires a complicated control system because the amplitude needs to be kept constant by using a detector for detecting the amplitude of a faint vibration period from a signal.